In wireless communication systems, space division multiple access (SDMA) has been actively considered in recent years as a technology for improving efficiency in the use of frequency, which is a limited resource. SDMA is a technology for using an adaptive array antenna 102 as shown in FIG. 9 to create beams having directivity toward terminals, and dividing a communication area using a single frequency into two or more wireless zones. SDMA technology makes it possible to avoid collisions between wireless signals transmitted and received between terminals 2a to 2c and a base station 101 even when using a contention method such as slotted Aloha to perform communication therebetween, thereby enabling two or more terminals to communicate with a base station at the same time on the same frequency channel.
Document 1, for example, discloses a method of further lowering the probability of failures in establishing a channel due to collisions between transmission signals when two or more terminals request the base station to establish a channel, by using an adaptive array antenna to dynamically control the directivity of the antenna.
On the other hand, “A DSRC System Proposal Extended From ARIB STD-T75 using Distributed Antenna and PSK-VP Scheme,” by H. Takai et al., ITST2002, pp. 239-244, November 2002, and Japanese Patent No. 3067751, for example, disclose road-to-vehicle communication systems that use a so-called distributed antenna technique in which antennas, each forming a wireless zone in a different area, are provided on the base station side, and transmit and receive the same data at substantially the same time on the same frequency to form an extended communication area.
In such road-to-vehicle communication systems, so-called diversity reception is used to perform selection/combining on signals received by two or more base station antennas based on error information in decoded data strings, reception levels, etc.
Patent document 1: Japanese Patent Application Publication No. 2002-300105
Non-patent document 2: “A DSRC System Proposal Extended From ARIB STD-T75 using Distributed Antenna and PSK-VP Scheme,” by H. Takai et al., ITST2002, pp. 239-244, November 2002
Patent document 3: Japanese Patent No. 3067751